1. Field of the Invention
The present invention relates to a thread cutting insert that is held on a tool body of an insert detachable type cutting tool to perform a thread cutting on a work.
2. Description of the Related Art
As this kind of thread cutting insert, various cutting inserts are suggested in JP-A-8-257837, JP-A-8-294804, JP-T-9-502396, and JP-T-2001-514089. In the various cutting inserts, a rake face is formed at each of corner portions of a triangular surface of an insert body that is formed in a triangular flat plate shape, and a pair of thread cutting edges formed in a convex V shape corresponding to a cross-section of a thread to be cut is formed on side edges of the rake face as viewed in a plane facing the rake face. Also, the inserts disclosed in JP-A-8-257837, JP-A-8-294804, JP-T-9-502396, and JP-T-2001-514089 are provided with convex protrusions projecting on the rake face as chip breaker.
These thread cutting inserts are generally used to cut a thread on a circumferential surface of a rotative work through a lathe turning of a plurality of cycles. That is, as viewed in the plane, the insert body is held on the tool body so that a bisector of a V-shaped projection formed by the pair of thread cutting edges as viewed in the plane is orthogonal to a rotation axis of the work. Then, while cutting into the circumferential surface of the work, a corresponding thread cutting edge is fed in plural cycles (plural times) by a predetermined feed per revolution in the predetermined direction parallel to the axis. In this case, threads having desired cross-sectional shape and root depth are obtained by gradually increasing amount of cutting for every cycle.
As described above, when cutting amount gradually increases for every cycle, radial infeed, flank infeed, or incremental infeed is employed as a cutting method. In the radial infeed, cutting amount increases so that the insert body is advanced along the bisector in each of the cycles. In the flank infeed, cutting amount increases so that the insert body is advanced to the front side of the feeding direction along the thread cutting edge provided on the rear side of the feeding direction of the pair of thread cutting edges. In the incremental infeed, cutting amount increases so that the insert body is alternatively advanced and retreated to the front and rear sides of the feeding direction with the bisector therebetween. Furthermore, in general, the radial infeed tends to be used for distributed thread cutting due to the versatility of the NC program of a machine tool.
In the radial infeed, since increase of the cutting amount is relatively large in the cycle between the anterior cycles in which cutting begins and the midterm cycles, among the plural cycles, a cutting chip produced in the radial infeed has a relatively thick thickness. Consequently, the cutting chip can be handled by curling thereof or by the convex protrusions disclosed in JP-A-8-257837, JP-A-8-294804, JP-T-9-502396, and JP-T-2001-514089. However, since increase of the cutting amount is relatively small in the last cycle of posterior cycles in which a thread is formed in a predetermined dimension and shape, a cutting chip has an extremely thin thickness. Consequently, a cutting chip, which is hardly curled and thus tends to extend, is produced in the posterior cycles including the last cycle, and the cutting chip is produced over the entire length of the pair of thread cutting edges.
Since the insert disclosed in JP-A-8-257837, JP-A-8-294804, JP-T-9-502396, and JP-T-2001-514089 has the convex protrusions for curling the cutting chip on the inside of the thread cutting edges formed in a V-shaped projection as viewed in the plane, it is effective to handle the relatively thick cutting chip between the anterior cycles in which cutting begins and the midterm cycles. However, the extremely thin cutting chip produced in the posterior cycles or in the last cycle is flown out at a speed to be cut from the work without resistance against the rake face. Accordingly, the cutting chip is flown out so as to easily pass over the convex protrusions and so as to extend. For this reason, it is not possible to control the discharge direction of the cutting chip, whereby the work with the thread formed in a predetermined dimension and shape is damaged or the cutting chip is entangled onto the work or the tool body.
In addition, among JP-A-8-257837, JP-A-8-294804, JP-T-9-502396, and JP-T-2001-514089, in particular, JP-A-8-257837 discloses an insert in which the convex protrusions is not provided on the rake face of the inside of the thread cutting edges and inclined surfaces forming saw-like unevennesses are provided on the rear side of the thread cutting edges. However, since the inclined surfaces are formed in one stage shape rising from the rake face to a boss surface, the inclined surfaces must have a high height to reliably curl the cutting chip produced in the posterior cycles by colliding against the inclined surfaces. In this case, since the cutting chip can be curled at one time and change the outflow direction, cutting resistance is caused to increase. Consequently, there is a possibility that chattering vibration is generated on the work or the tool body, or accuracy of the finished surface deteriorates.